As an information recording medium adapted to be mounted in a magnetic disk device such as a hard disk drive (HDD), there is a magnetic disk. The magnetic disk is produced by forming a thin film such as a magnetic layer on a substrate and, conventionally, an aluminum substrate has been used as such a substrate. Recently, however, in response to the pursuit of higher recording density, the occupation ratio of glass substrates, which can shorten the distance between a magnetic head and a magnetic disk as compared with the aluminum substrate, has been gradually increasing. In order to minimize the flying height of the magnetic head, a glass substrate surface is precisely polished, thereby achieving higher recording density. In recent years, the demand for an increase in HDD recording capacity and a reduction in HDD price has been increasing more and more. In order to achieve this, higher quality and lower cost are required also for magnetic disk glass substrates.
High smoothness of a magnetic disk surface is essential for a reduction in the flying height of a magnetic head which is necessary for achieving higher recording density as described above. In order to obtain the high smoothness of the magnetic disk surface, a highly smooth substrate surface is, after all, required and thus it is necessary to precisely polish a glass substrate surface.
A conventional glass substrate polishing method is carried out using polyurethane-based polisher polishing pads while supplying a slurry (polishing liquid) containing a polishing abrasive of metal oxide such as cerium oxide or colloidal silica. A glass substrate with high smoothness can be obtained, for example, by polishing using a cerium oxide-based polishing abrasive and then by finish polishing (mirror polishing) using colloidal silica abrasive particles. Herein, for example, it is proposed to use a colloidal silica slurry, which is pH-adjusted to be acidic, for polishing a magnetic disk substrate (see JP-A-H07-240025 (Patent Document 1)). It is also proposed to use a colloidal silica slurry, which is adjusted to have a pH more than 10.2 and not more than 12 by adding alkali to a polishing liquid, for polishing a magnetic disk glass substrate (see JP-A-2003-173518 (Patent Document 2)).